In displays and integrated circuits such as plasma display panels (hereinafter abbreviated as “PDP”), field emission displays, liquid crystal displays, fluorescent displays, ceramic laminated devices, and hybrid integrated circuits, substrates are used that have electrodes and wirings formed of, for example, Ag or Cu on the surfaces thereof. Such electrodes and wirings may be covered with insulating glass materials for protection. A PDP, which is a typical display, is described below as an example.
Generally, a PDP is configured to include two opposing glass substrates that are provided with a pair of electrodes arranged regularly, with gas that mainly contains inactive gas, such as Ne or Xe, being sealed therebetween. In the PDP, voltage is applied between electrodes to allow electric discharge to be generated in minute cells located around the electrodes, so that each cell is allowed to emit light and thereby display is performed. These electrodes are covered with an insulating material called a dielectric layer for protection.
For instance, in a glass substrate to serve as the front panel of an AC type PDP, transparent electrodes are formed and electrodes of metal, such as Ag, Cu, or Al, with lower resistivity are formed further thereon. A dielectric layer is formed covering those combined electrodes, and a protective layer (MgO layer) is formed further thereon.
The dielectric layer to be formed covering the electrodes can be a thin film of, for example, SiO2 formed by a method such as CVD. Usually, however, from the viewpoints of equipment and cost, glass with a low softening point is used. The dielectric layer is formed by applying a paste containing glass powder to cover electrodes by, for instance, a screen printing method or a die coating method, and then baking it.
The characteristics required of a glass composition that forms a dielectric layer include, for example:    (1) having insulation properties because it is formed on electrodes;    (2) having a thermal expansion coefficient that is not greatly different from that of the substrate material so as to prevent the glass substrate from warping and the dielectric layer from peeling off and cracking in the case of a large-area panel;    (3) being amorphous glass with a high visible light transmittance so as to use efficiently the light generated from phosphors as display light when being used for a front panel; and    (4) having a lower softening point so as to conform to the heat resistance of substrate glass.
Examples of the glass substrate to be used for a PDP include soda lime glass, which is window sheet glass that is produced by a float process and is generally readily available, and glass with a high strain point developed for PDPs. They usually have a heat resistance up to 600° C. and a thermal expansion coefficient of 75×10−7 to 85×10−7/° C.
Accordingly, with respect to the item (2) described above, the thermal expansion coefficient is desirably about 60×10−7 to 90×10−7/° C. With respect to the item (4) described above, since it is necessary to bake the glass paste at 600° C. or lower, which is the strain point of the glass substrate, the glass composition needs to have a softening point of 595° C. or lower, more desirably about 590° C. or lower so as to be softened enough even if the glass paste is baked at a temperature of 600° C. or lower.
Currently, PbO—SiO2 glass whose main raw material is PbO is used mainly as a glass material that satisfies the requirements as described above.
However, environmental concerns in recent years require dielectric layers that are free from Pb. Furthermore, glass materials are required to have even lower permittivity in order to reduce power consumption of PDPs. For example, a Bi2O3—B2O3—ZnO—SiO2 glass material (see, for example, JP 2001-139345 A) having a lower softening point achieved by containing zinc borate as a main component thereof and Bi instead of Pb has been developed as glass that is free from Pb. However, like the Pb material, the Bi material also has a problem in that its relative permittivity is as high as about 9 to 13. At present, there are demands for materials with distinctly lower permittivity than that thereof, for example, materials with a relative permittivity of 7 or lower, more desirably 6 or lower.
Therefore, materials also are proposed that have attained a relative permittivity of around 7 by using the zinc borate glass (alkali metal oxide-B2O3—ZnO—SiO2 glass) containing alkali metal instead of Pb in order to obtain both a low permittivity and a low softening point (see, for example, JP 9 (1997)-278482 A, JP 2000-313635 A, and JP 2002-274883 A).
However, in the alkali zinc borate glass that has been studied conventionally, the lowest relative permittivity is only 6.4. Furthermore, although it is allowed to have a low softening point, suitable thermal expansion coefficient, and low permittivity at the same time, glass having a high glass transition temperature (glass transition point) in addition thereto has been difficult to obtain.
If the glass to be obtained is one for simply covering electrodes, it is enough to allow it to have a low softening point, a suitable thermal expansion coefficient, and a low permittivity. However, in the case of PDPs, after the electrodes are covered with glass, the glass layer is heated again at a temperature of nearly 500° C. in, for example, the step of annealing a MgO layer and the sealing step of joining a front panel and a rear panel to each other. Since the softening point of the glass for a dielectric layer is a little lower than 600° C., it will not necessarily be softened even if it is heated at a temperature of about 500° C. However, if this heating temperature exceeds the glass transition temperature considerably, the thermal expansion coefficient will increase rapidly. Accordingly, especially, in a large area display, a dielectric layer will separate from a substrate or will crack, which results in a decrease in insulation and reliability. According to the studies made by the inventors, in order to reheat-treat the glass at about 500° C., the glass transition temperature required of the glass is desirably at least 465° C. and more desirably at least 480° C. Similarly in, for example, displays other than PDPs and circuit boards, when electrodes and wirings are covered with a glass material and thereafter the glass material is heat-treated at a high temperature again, there was a risk of similar problems arising.
According to the studies made by the inventors, in order to allow alkali zinc borate glass to have a low permittivity, it is necessary to increase the amount of boron, but an increase in the amount of boron tends to lower the glass transition temperature. In conventional glass for covering electrodes, attention was not paid to the glass transition temperature at all. Accordingly, a material having a high glass transition temperature as well as a low softening point, a low permittivity, and a suitable thermal expansion coefficient has not been obtained.
Furthermore, in alkali glass containing a large amount of boron, there also is a problem in that a heat treatment tends to cause components to sublimate/evaporate. This evaporation phenomenon significantly occurs mainly between the glass transition temperature and the softening temperature and thereby evaporated components may adhere to other places on the substrate to deteriorate the insulation properties or may enter the protective film (MgO layer) formed on the dielectric layer to deteriorate the properties of the protective layer.